1. Field of the Invention
The invention relates to semiconductor wafer processing and to systems and methods that measure wear on polishing pads used in chemical mechanical polishing.
2. Description of Related Art
Chemical mechanical polishing (CMP) uses a chemical reactant in a polish or slurry and the mechanical action of a polishing pad to planarize a wafer. The polishing removes material from the surface of the wafer but also wears away the polishing pad. Eventually, the polishing pads must be replaced because the pads have worn out or worn unevenly. Unevenly worn pads can cause uneven removal of material from a wafer and may result in a poorly polished surface. Accordingly, for efficient operation of a CMP system, polishing pads must be regularly checked for wear and/or replaced. Detaching a pad to measure pad thickness is impractical because pads are typically attached with an adhesive that would tear and make the pad unusable. Optical measurement of an attached pad is possible, but a non-uniform distribution of water and slurry on the pad prevents optical interferometry as a viable technique for thickness measurements unless the system is stopped and the pads are cleaned and dried.
A concern in current CMP systems is proper scheduling of maintenance of the polishing pads. Due to the difficulties in accurately measuring polishing pads, a current practice is to replace pads after polishing a set number of wafers. However, the number of wafers that can be polished before pad replacement is required may vary because the properties of the pad material can vary from lot to lot and different wafers and polishing parameters cause pads to exhibit a wide range of wear rates. Replacing polishing pads too often increases down time for the CMP system and reduces the maximum yield of polished wafers. A CMP system is desired that minimizes down time for polishing pad maintenance but still avoids problems associated with worn out or unevenly worn polishing pads.